Can't solve Towers of Hanoi in PDDL
Ah hah!

The way I had defined the disks made d5 the LARGEST disk, not the smallest. So, the last few lines of the file should be:

   (clear d4) (clear d2) (clear d5)
    (on d4 peg1) (on d1 peg2) (on d2 d1) (on d3 peg3) (on d5 d3))

  (:goal (and (on d1 d2) (on d2 d3) (on d3 d4) (on d4 d5)))
)


However

If I wanted the opposite to be true, d1 to be the largest, the changes would have been as following:

; Swapped ?disc &amp; ?to in (smaller ...) statement
:precondition (and (smaller ?disc ?to) (on ?disc ?from) 


And

; Swapped all d* and peg* objs within (smaller ...) staements
(smaller d1 peg1) (smaller d2 peg1) (smaller d3 peg1)
(smaller d4 peg1) (smaller d5 peg1)
(smaller d1 peg2) (smaller d2 peg2) (smaller d3 peg2)
(smaller d4 peg2) (smaller d5 peg2)
(smaller d1 peg3) (smaller d2 peg3) (smaller d3 peg3)
(smaller d4 peg3) (smaller d5 peg3)


:)